In photolithography processes, light is passed through a mask having a desired pattern onto a light-sensitive substrate. Diffraction effects can degrade the replication when the mask includes fine-scale features comparable to the wavelength of light, typically on the order of one fourth (1/4) to one half (1/2) micrometer. Recently, much attention has been focused on the use of X-rays for lithography applications since X-rays have much shorter wavelengths. Thus, X-ray lithography is one of the leading techniques being considered for producing the next generation of micro-electronic components.
Several sources for the production of X-rays for use in lithography have been developed. Electron-impact X-ray sources generally produce high-energy X-rays which are not stopped in the resist layer and can continue on to damage the substrate material. Synchrotron sources emit softer X-rays but are limited by high cost and large physical dimensions. Laser produced X-rays have been tested but have thus far yielded poor performance and efficiency.
It is generally known that X-rays having high light-to-X-ray conversion efficiencies can be produced if four criteria are satisfied: (1) the laser pulse intensity is greater than approximately 6 TW/sq cm. at the focussed spot on the target; (2) the focal spot is not so small that the expansion of the plasma causes rapid cooling; (3) the wavelength of the laser radiation is short enough to optimize the coupling between the laser pulse and the plasma target; and (4) an appropriate target material is chosen.
The criteria outlined above may be met by more than one type of laser, including solid state and discharge-pumped rare-gas-halide lasers. Discharge-pumped rare-gas-halide lasers, more commonly referred to as excimer lasers, produce pulses of shorter wavelength than solid-state lasers and allow more efficient coupling to the plasma target. Unfortunately, the pulses produced are generally too low in energy and too long in duration to give the instantaneous peak power required to produce a sufficiently high plasma temperature. As a result of low peak power, prior systems which generate X-rays using discharge-pumped lasers have yielded poor conversion efficiency. A laser of this type would require compression of the output pulse by an order of magnitude to produce the instantaneous intensity required for adequate light-to-X-ray conversion efficiency.
Presently, excimer lasers having pulse widths in the nanosecond range are available. However, the power output of these short pulse lasers is too low for generation of a plasma which will emit X-rays useful in lithography applications. On the other hand, the high power lasers produce pulses which are too long in duration to be useful in X-ray generation. It is generally believed that nanosecond length pulses are required for generation of X-rays from a plasma to allow the laser power to be put in the plasma before the plasma expands excessively.